Organic growers target nitrogen balance for crops ByBob Johnson
happens are soil temperature, soil mois- ture and thequalityof theorganic source.” This processmovesmore quicklywhen the soil iswarmandmoist. A lower ratioof carbon tonitrogenalso speeds thisminer- alization process, whichmeans nitrogen becomes available fastest inmost growing areas in the summer. “In the summer, the soils arewarm, and they are moist from irrigation,” Geisseler said. “In the winter and spring, the tem- perature is limiting. In the fall, moisture is limiting if the field is fallow.” Nitrogenmineralizes slower fromcover crops and residuewhen the percentage of nitrogen is low. “Broccoli, lettuceand legumesarehigh- er in nitrogen, and tomatoes, melons and young cereal cover crops are lower in ni- trogen,” Geisseler said. That difference impactswhat cover crop varieties to plant—and how to manage cover crops at termination and crop resi- due after harvest. “Legumeandmustardcover crops tend to have a higher percentage of tissue ni- trogen,” said Louise Jackson, a UC Davis plant sciences professor emeritus who did pioneering studies on organic nitro- gen management. “If you have a legume cover crop, youshouldavoida lagbetween incorporation and planting the cash crop See NITROGEN, Page 11
Organicvegetableandberrygrowersface agreatchallengeknowinghowmuchnitro- genwillbeavailabletotheircropandwhen. While conventional growers apply syn-
thetic fertiliz- ers and know w i t h s o m e certainty how much nut r i - ent s wi l l be
available to the crop, organic growers rely on a variety of ways to build soil with nu- trients that becomeavailable slowlyover a period of months and even years. “A lot of the ni trogen from cover crops, crop residue and compost is be- ing built into the soil, and that is where we get this tremendous ongoing release of nitrogen in organic systems,” said Margaret Lloyd, University of California CooperativeExtensionsmall farmsadvisor inWoodland. “With cover crops, residue andcompost,webuildthis robustnitrogen source in our soil.” Hitting the target in applying nitrogen fertilizer is important foreconomicanden- vironmental reasons but is challenging in organic systemsbecausemost of thenitro- genbecomes availablegradually, she said. “It’s very difficult to precisely apply ni- trogen in organic systems,” said Richard Smith, UCCE farm advisor in Salinas.
Broccoli grows at Lakeside Organic Gardens in Watsonville. Applying nitrogen is challenging for organic farmers, who look to build soil with nutrients slowly added over months or even years.
reallymany sources: the soil, cover crops, poultrymanure, compost, granular fertil- izer, the irrigation water,” Lloyd said. “All of the nitrogen contributions frommany places are building the nitrogen program in an organic system.” Micro-organisms in the soilmakenitro- gen available to the crop through a com- plexprocess thatdependsonmany factors, including theweather. “When you add crop residue or organ- ic amendments, the nitrogen needs to be converted to ammoniumor nitrate by micro-organisms,” said Daniel Geisseler, UCCE specialist innutrientmanagement. “The main factors affecting how fast this
“Improvingnitrogen fertilizer applications will revolve around growers’ experience and skills, soil-nitrate testing and under- standing the crop growth characteristics.” Lloyd and Smithmade their remarks as UCCE advisors and specialists presented their advice on matching available nu- trients with crop needs in presentations during the virtual Organic Fertility Series. Presentations by the seven experts on the role of cover crops, compost and crop residue in building fertile organic soil are available through the Monterey County Cooperative Extensionwebsite. “Wemay thinkof amendments in terms of their contribution to nitrogen, but it’s
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6 Ag Alert September 8, 2021
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